1. Field of the Invention
The present invention relates to improved catalysts for the treatment of exhaust gases from spark ignited internal combustion engines, and, more especially, to the preparation of such catalysts for the treatment of exhaust gases from internal combustion engines, which catalysts effect not only the oxidation of carbon monoxide and of the hydrocarbons present in said exhaust gases, but also, optionally simultaneously, the reduction of the nitrogen oxides present in these gases.
2. Description of the Prior Art
In general, the catalysts suitable for the catalytic oxidation and optionally for the catalytic reduction of the constituents present in the exhaust gases from spark ignited internal combustion engines comprise a support or carrier which is coated and/or impregnated with at least one precious metal from the platinum group, such as platinum, rhodium, ruthenium, palladium or iridium, and at least one base metal, these base metals typically being selected from among: cobalt, zinc, manganese, nickel, tungsten, cerium, iron, copper, neodymium, praseodymium, rhenium, yttrium, magnesium, chromium, zirconium, molybdenum, lanthanum, tin, calcium, strontium and barium.
Such catalysts are typically prepared by conventional technique either by impregnation of the support or by the introduction of the metals of the active phase during the manufacture or production of the support (carrier). In general, this is done by impregnation of the support with solutions of inorganic or organic compounds of the metals sought to be introduced.
Thus, according to published European Patent Applications Nos. 27,069, 54,472 and 60,740, assigned to the assignee hereof, it is known to produce multifunctional catalysts for the treatment of exhaust gases from internal combustion engines either by (i) successive impregnation of the support with a solution containing compounds of iron and of cerium, and then with a solution containing the compounds of the precious metals of the platinum group sought to be incorporated therein, or by (ii) impregnation of the support with a solution containing a mixture of the salts of the base metals and of the precious metals sought to be incorporated. In order to obtain a high initial activity, it is generally advantageous to carry out a reduction of the precious metal compounds under hydrogen during the preparation of these catalysts. Cf. U.S. Pat. Nos. 4,378,307 and 4,426,319, also assigned to the assignee hereof.
It too is possible, according to French Pat. No. 2,196,195, to effect reduction of small amounts of the catalyst with compounds such as hydrazine which are much stronger reducing agents than hydrogen. Such a reduction is carried out at a temperature ranging from 80.degree. to 250.degree. C., preferably from 100.degree. to 200.degree. C. The process described in this '195 patent, however, is stated to be inapplicable for the treatment of large amounts of catalyst. On the other hand, reducing agents of this type are indeed quite strong, effecting the reduction of cerium dioxide (CeO.sub.2), for example, when it is present in the active phase. Consequently, catalysts prepared by this particular process do not possess adequate initial activity.